The present invention relates to a developing agent used in, for example, an electrophotographic apparatus, particularly, to a negative developing agent.
It is known to the art to use a charge control agent in a magnetic toner. However, since the magnetic toner is superior to the nonmagnetic toner in its charging properties and transfer properties because of its magnetic characteristics, the magnetic toner can be developed even if a charge control agent (CCA) is not contained in the toner. Therefore, a magnetic toner that does not contain a CCA is preferred in recent years in view of reduction of the manufacturing cost and adverse effects given by the CCA to the environment and the human body.
However, when it comes to a magnetic toner that does not contain a CCA, the rise of charging is poor under an environment of a high temperature and a high humidity, giving rise to the phenomenon that the solid image is thinned. Also, when it comes to the life of the toner, the charging amount distribution becomes broad on the side of the large charging amount, giving rise to blurring of the printed character and a poor transference.
When it comes to, for example, a negative magnetic toner, the toner is mixed with a positive magnetic powder so as to allow the charge of the highly charged toner to leak to, for example, the carrier and to the developing device, so as to prevent the highly charged side from becoming broad.
Since the magnetic powder exhibits a strong agglomeration capability, the magnetic powder is unlikely to be dispersed on the surface of the toner when the toner is added to and mixed with the toner. Also, even if once dispersed, the magnetic powder is liberated from the toner so as to be present independently. In such a case, since the magnetic powder is positive, which is opposite to the polarity of the toner, the magnetic powder is developed in the non-image portion of the photoreceptor. What should also be noted is that magnetite, ferrite or the like is used in general as the magnetic powder. Since such a magnetic powder is black, the magnetic powder appears as a fogging after the transference. Since the size of the magnetic powder developed in the non-image portion of the photoreceptor is small, the magnetic powder passes through the blade when the magnetic powder is present together with the toner, making it difficult to remove the magnetic powder by cleaning with the blade. It follows that, during the life of the developing agent, the magnetic powder tends to bring about filming within the photoreceptor.
An additional difficulty inherent in the prior art is that the flowability of the toner is impaired so as to increase the amount of the toner remaining inside the cartridge.
An object of the present invention, which has been achieved in view of the situation described above, is to provide a developing agent satisfactory in the charging at start-up and in the stability of the charging over the entire life of the developing agent, and capable of forming a satisfactory image free from blurring of the character and a defective transference and having a sufficiently high image density.
Another object of the present invention is to provide an image forming apparatus capable of forming a satisfactory image free from blurring of the character and a defective transference and having a sufficiently high image density by using a developing agent satisfactory in the charging at start-up and in the stability of the charging over the entire life of the developing agent.
According to a first aspect of the present invention, there is provided a negative developing agent comprising a carrier and a negative magnetic toner including a negative magnetic toner particle containing a magnetic powder and a binder resin; a negative conductive silica; an inorganic oxide, and not containing a charge control agent.
According to a second aspect of the present invention, there is provided a negative developing agent comprising a negative toner particle containing a coloring agent and a binder resin; a negative conductive silica; and a negative magnetic toner containing an inorganic oxide but not containing a charge control agent.
Further, according to a third aspect of the present invention, there is provided an image forming apparatus, comprising:
at least one image carrier;
a developing device having a hollow cylindrical rotatable sleeve for holding a developing agent and a developing roller including a magnetic roller having a plurality of magnetic poles and rotatable independent of the sleeve, the developing agent including a carrier and a negative magnetic toner containing a negative magnetic toner particle having a magnetic powder and a binder resin, a negative conductive silica, an inorganic oxide, not containing a charge control agent;
a transfer device,
a cleaning device, the developing device, transfer device and cleaning device being arranged to face the image carrier in the order mentioned; and
a fixing device arranged downstream of the transfer device and having a pair of fixing rollers.
The developing agent of the present invention is satisfactory in the charging at start-up and in the stability of the charging over the entire life of the developing agent and is capable of forming a satisfactory image free from blurring of the character and a defective transference.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.